1. Field of the Invention
This invention relates to a composition and method useful in fabricating high temperature degradation resistant rigid foams and is more particularly concerned with a composition and method useful in fabricating thermally stable urethane-free oxazolidone-modified isocyanurate foams and to the foams so prepared.
2. Description of the Prior Art
The formation of cellular polymers from polyisocyanates in the presence of epoxides has been described previously. For example, Nicholas and Gmitter, Journal of Cellular Plastics, pages 85 to 90, January 1965, (see also French Pat. No. 1,441,565) describe the polymerization of a polyisocyanate or an isocyanate-terminated prepolymer, preferably in the presence of a polyol, using as catalyst a minor proportion of a polyepoxide and certain tertiary amines. In this instance the combination of polyepoxide and tertiary amine is employed in catalytic amounts, the main reaction being the polymerization of the polyisocyanate to form a polyisocyanurate.
Hoy, U.S. Pat. No. 3,198,851 describes the "one-shot" preparation of non-cellular products by reacting a polyisocyanate and a polyepoxide, in proportions which include substantially stoichiometric proportions, in the presence of a boron trifluoride-tertiary amine complex. The reaction mixture is heated to a temperature at which free boron trifluoride is liberated from the complex in order to initiate the reaction. No teaching is made of the one-shot preparation of cellular polymers by the above reaction.
McGary et al, U.S. Pat. No. 3,242,108 describes the preparation, inter alia, of cellular polymers by reacting a polyisocyanate and a polyepoxide in the presence of a low molecular weight polyol, usually an aliphatic diol or triol, and a boron trifluoride-amine complex as catalyst. The resulting cellular polymers contain substantial proportions of polyurethane linkages, derived by reaction of the polyisocyanate with the polyol, and are accordingly limited thereby in their ability to resist deformation on exposure to high temperatures.
Schramm, U.S. Pat. No. 3,313,747 describes the preparation of polymerizable isocyanato-substituted 2-oxazolidinone by reacting an excess of polyisocyanate with a polyepoxide in the presence of a quaternary ammonium halide and a low molecular weight primary or secondary monohydric alcohol. The polymerizable isocyanato-substituted 2-oxazolidinone is then converted to a cellular polymer by reaction with water.
Ashida et al, U.S. Pat. No. 3,793,236 discloses oxazolidone-modified isocyanurate resins prepared by a pre-polymer of "two-shot" method; including the steps of (a) reacting a polyisocyanate with a polyepoxide in the presence of an oxazolidone-formation catalyst to form an isocyanate-terminated polyoxazolidone and then (b) trimerizing the isocyanate of the isocyanate-terminated polyoxazolidone in the presence of a trimerization catalyst to produce an oxazolidone modified isocyanurate resin. Comparision Formulation C described in Example 1, Table II at column 9 of the '236 patent described therein as a "one-shot" formulation containing 9 parts by weight of a polyepoxide (EPON 828) based on 100 parts by weight of polyepoxide and polyisocyanate (EPON 828 plus ISONATE 135). The friability of a "one-shot" foam prepared using Formulation C is disclosed in Table II of the '236 patent as being very high (68 percent weight loss) as compared to an analogous, but "two-shot" foam prepared using Formulation A from Table II of the '236 patent.
Hayash et al, U.S. Pat. No. 3,673,128 discloses polyoxazolidone foams produced by reacting a polyepoxide with a polyisocyanate in a proportion of from about 0.70 equivalent to about 1.25 equivalents of polyepoxide per equivalent of polyisocyanate. However, the use of such high relative amounts of polyepoxide is costly since commercial polyepoxides are about twice as expensive as polyisocyanates. Moreover, the use of these ratios of polyepoxide to polyisocyanate have been found to produce such a strong exothermic reaction as to cause scorching and associated thermal degradation of interior portions of the foam during fabrication thereof.